Coating to reduce coking and assist with decoking in transfer line heat exchanger

ABSTRACT

Coke formation in pyrolysis furnaces is controlled by applying a coating of boron nitride to pyrolysis furnace process equipment surfaces, for instance, parts of the transfer line heat exchanger assembly.

FIELD OF THE INVENTION

The invention relates generally to a method of inhibiting coke or carbonformation and allowing easier cleaning of metal surfaces of processingequipment during high temperature processing of hydrocarbons. Moreparticularly, the invention relates to the coating of certain surfacesof a transfer line heat exchanger with a boron-nitride composition toreduce coke formation and allow easier cleaning of those surfaces.

BACKGROUND ART

In traditional pyrolysis processing using pyrolysis furnaces, mixturesof hydrocarbons and steam flow through long coils or tubes which areheated by combustion gases to produce olefins, such as ethylene andpropylene, as well as other valuable by-products. Heat is transferredfrom the hot combustion gases to the hydrocarbon feedstock passingwithin the coils. The hydrocarbon feedstock is heated within the coilsto temperatures typically in the range of about 750° to 950° C. to formthe product stream.

After passing through the pyrolysis furnace, the product stream istypically cooled or “quenched” in a transfer line heat exchanger (TLE)to both stop the reaction, and to cool for processing and separation. ATLE is designed to recover sensible heat from the hot product streamleaving the pyrolysis furnace. Heat is transferred from the hot productmixture to low pressure steam in the TLE to form high-pressure steam.

Coke formation is a traditional problem in the TLE as hydrocarbons aredehydrogenated, forming a solid residue on the metal and refractorysurfaces of the hot product side of the TLE. Coke formation andcollection in the TLE typically results in poorer heat transfer, whichin turn results in decreased production of high-pressure steam. Cokeformation in the TLE also often results in a larger pressure drop acrossthe TLE. This problem is particularly acute in the inlet cones of theTLE.

The typical operating cycle for a TLE is to operate for a period of timecooling the product stream from the pyrolysis furnace. During thisoperation coke forms in the inlet cone plugging up the tubes and tubesheet of the TLE and restricting flow. When the pressure drop becomestoo high, the TLE will be hot cleaned (decoking cycle) using steaminjected into the inlet cone to remove coke and open up the tubes of theTLE so more product gas can flow and there is less pressure drop. Thisis only partially effective and after two to four cycles of operationand hot cleaning, the TLE inlet cone must be removed so the coke can beremoved through more aggressive methods. This process of removal of thecoke on the TLE inlet cone is typically accomplished mechanically,usually entailing hammers and chisels, which also damages the inlet conerefractory and the tubesheet of the TLE. Then, the operation cycle isstarted over. Because of the damage done to the inlet cone refractory ofthe TLE by the mechanical removal of coke, the inlet cone refractorymust be repaired or replaced at a significant cost.

In addition to the shutdown and startup process of the pyrolysisfurnace, the mechanical de-coking operation of the TLE itself frequentlyrequires several days. De-coking therefore results in increased downtimerelative to olefin production time, frequently amounting to a severalpercent loss of olefin production during the course of a year. De-cokingis also relatively expensive and requires appreciable labor and energy.

Previous methods have been used to control coke formation. For instance,coke inhibitors, i.e., chemical additives, or special coatings of metalsurfaces which suppress coke formation have been used. Cokeinhibitors/surface coating act to passivate catalytically active metalsites through chemical bonding interactions, and/or forming a thin layerto physically isolate metal sites from coke precursors in the processstream, and/or interfering with those free radical reactions leading tocoke formation by blocking active sites on surfaces. Such additives areexpensive and may lead to product gas stream quality issues.

What is needed is a method of controlling the growth and formation ofcoke, particularly in the inlet cone and tube sheet of the TLE. What isfurther needed is a method to reduce the damage done to the TLE inletcone and tube sheet during a de-coking operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of coke formation on a metal surface of the TLE.

FIG. 1A is a pictorial depiction of filamentous coke.

FIG. 1B is a pictorial depiction of filamentous and free radical coke.

SUMMARY

The methods described herein relate generally to the field of reducingand controlling coke formation in the TLE during the olefin productionprocess.

In one embodiment of the present disclosure, a method of controllingcoke formation in pyrolysis furnace process equipment is described,wherein the surface of the process equipment is coated with a layer ofboron nitride.

In another embodiment of the present disclosure, a method of controllingcoke formation in a transfer line heat exchanger is described, wherein aboron nitride paint is formed by combining dry boron nitride powder withdistilled water. The boron nitride paint is applied to a transfer lineheat exchanger tube sheet.

In still another embodiment of the present disclosure, a boron nitridepaint for coating the surfaces of pyrolysis furnace process equipment isdescribed which includes providing a dry boron nitride powder. The dryboron nitride powder contains boron nitride with a hexagonal crystallinestructure. Distilled water is provided and the boron nitride powder ismixed with the distilled water to form the boron nitride paint. Theboron nitride paint comprises between 20 and 45% boron nitride byweight.

DETAILED DESCRIPTION

While not bound by theory, applicants have determined that coke isclassified into two types: Catalytic coke and Pyrolytic coke. Catalyticcoke is formed by dehydrogenation of hydrocarbon with catalytic actionof metal components on the surface. Metal components, such as nickel andiron, may catalyze a hydrocarbon to reduce or eliminate hydrogen fromthe olefin, termed “dehydrogenation.” Metal components presentingcatalytic activities are generally in the order of Ni>>>>Fe>>Cr, NiO>Ni,FeO>Fe>Fe2O3. These metals and oxides catalyze reaction to form filamentand coil type coke by successive dehydrogenation, as shown in FIG. 1.

Catalytic coke tends to be very mechanically hard (“hard coke”) andnormally difficult to remove. Hard coke must often be removed fromsurfaces by mechanical means. The formation of catalytic coke isbelieved to be most often involved in beginning the coking process ofthe TLE and is believed to act as trap for pyrolytic coke

Pyrolytic coke is divided into gaseous and condensation coke. Pyrolyticcoke is softer and generally easier to remove than catalytic coke.Gaseous coke is typically formed by dehydrogenation of such lightolefinic hydrocarbon as acetylene. Condensation coke is formed bycondensation, polymerization, and dehydrogenation of heavy aromaticcompounds. Pyroltyic coke can be classified as globular, black mirror,fluffy or amorphous types according to morphology.

It is believed that the coking process of the TLE begins with thehydrocarbon reacting by catalyst action of metal components on metalsurfaces and forms filamentous coke, which grows and provides depositsites for various types of coke. Free radical coking causes cokefilaments to thicken and, as catalytic coke filaments grow, carbonstarts to block metal surfaces. Tar is formed as condensation collectsin the filaments. The filaments formed by catalytic coking stop growingwhen metal particles are covered with carbon and, afterwards, radicaland condensation coking become dominant.

FIG. 1 depicts one example of coke formation on typical metal surface 20of pyrolysis furnace TLE 10. Filamentous coke filaments 30 growoutwardly (as shown by upward arrow 35) from typical metal surface 20.Free radical coke 40 tends to grow from filamentous coke filaments 30,causing filaments 30 to grow and thicken. Filamentous coke filaments 30tend to continue to grow outwardly (as shown by horizontal arrows 45)until active metal site 50 is blocked by free radical coke 40, asindicated by blocked metal site 60. FIG. 1A is a pictorial depiction offilamentous coke during initial formation. FIG. 1B is a pictorialdepiction of filamentous coke and free radical coke formation.

In the pyrolysis furnace, the product gas includes the coke precursors.When the product gas is quenched to stop the reaction in the TLE beforemain fractioner, coking is common on gas entry refractory cone wall, theTLE tube (inside) wall and tubesheet surface. This coking traditionallycauses two problems: pressure drop and spalling, i.e., the breaking ofthe coke into small particles. Either of these problems can result in toreduced product gas flow and ultimately a shutdown for decoking.

In certain embodiments of the present disclosure, a coating is appliedto the TLE tubesheet and inlet cone refractory and other elements toreduce the amount of catalytic hard coke formed during pyrolysis furnaceoperation. In at least some embodiments of the present invention, thecoating applied to the TLE tubesheet and inlet cone refractory isresistant to temperatures of 1600° F. (870° C.) or higher, is easy toapply to existing installations (retrofitability). In these embodiments,the coating adheres well to metal and/or refractory surfaces and iseffective as a thin film or layer. In certain embodiments of the presentdisclosure, the coating is water based; in other embodiments the coatingis an organic solvent based material. In certain embodiments, thecoating can be readily sprayed onto surfaces at room temperature. It ispreferable that the coating be easy to dry and easy to cure developinggood adhesion or bonding to metal and/or refractory surfaces. Typically,the coating requires minimum surface preparation.

In certain embodiments of the present invention, the coating material isboron nitride. Boron nitride has a number of crystalline structures,includes hexagonal, cubic, and wurtzite. In certain embodiments of thepresent disclosure, the TLE coating includes hexagonal boron nitride.

The hexagonal crystalline structure of boron nitride is oftenlubricious, exhibiting some of the same properties of solid lubricantsas graphite and molybendum disulfide, with low shear strength, lowabrasivity, a good adherence of solid lubricant film and superiorthermal stability. Hexagonal boron nitride typically has an oxidationthreshold of approximately 1562° F. (850° C.) in an oxidizing atmosphereand up to 1832° F. (1000° C.) in a reducing atmosphere. In certaincircumstances, hexagonal boron nitride can be used in inert or vacuumatmospheres at temperatures of approximately 3632° F. (2000° C.).Hexagonal boron nitride tends to have a high thermal conductivity with alow thermal expansion. Hexagonal boron nitride typically has arelatively high thermal shock resistance compared to other oxide basedrefractory compounds. and tends to be chemically inert to the compoundsto which it is exposed in a pyrolysis furnace. It therefore tends tohave a relatively high resistance to chemical attack compared to otheroxide based refractory compounds. and is resistant to chemicalcorrosion. Boron nitride tends to have an excellent parting planecompared to oxide based refractory compounds and reduces sticking inglass forming applications.

As those of skill in the art will appreciate, the thickness of thecoating on the TLE inlet cone components and tubesheet depends on anumber of factors including the cost of the coating material and theporosity and/or roughness of the surface to which it is applied. Incertain embodiments of the present disclosure, the thickness of thecoating applied to the TLE is in the range of about 25 microns to about100 microns; in certain other embodiments of the present disclosure, thethickness of the coating is between about 40 microns and about 60microns. In still other embodiments the thickness of the coatingmaterial is about 50 microns. In certain embodiments of the presentdisclosure, the thickness of the coating is not uniform, but variesacross the tubesheet or inlet cone refractory surface, depending on thecharacteristics of the surface and the expected level of coke formation.

Application of the boron nitride coating may be accomplished bytraditional coating application methods. In certain embodiments, awater-based boron nitride paint is made from dry boron nitride powderand water, such as distilled water. In particular embodiments, thewater-based boron nitride paint is at 20 to 40% solids concentration byweight; in other embodiments, a 20 to 35% concentration is used,although as those of ordinary skill in the art will understand with thebenefit of this disclosure, a greater or lesser concentration of solidsmay be used.

In certain embodiments of the present disclosure, the boron nitridecoating is accomplished by use of a spray gun, such as a DeVilbissCompact Pressure Spray Gun, although this example is non-limiting andany suitable spray gun may be used.

When applying the boron nitride coating, it is preferable to have thesurface to which the coating is to be applied to be clean, dry, and asfree from grease or oil as is practical. In certain embodiments of thepresent disclosure, the TLE inlet cone and tubesheet have not beenpreviously used, i.e., they have not been exposed to process chemicals.In such embodiments, it may be necessary to roughen the surface of thearea to which the coating is to be applied in order to assist inphysical adherence to the surface. Non-limiting examples of surfaceroughening include those specified by SSPC-SP6 and NACE 3-CommercialBlast Cleaning.

In certain other embodiments, such as when the TLE has been previouslyused in a pyrolysis furnace, the residual coke on metal surfaces such asthe TLE tubesheet inlet surface and Intrabody Flow Diverter surface arefirst removed and cleaned by a hydroblasting procedure, typically atabout 10,000 psi then allowed to air dry. In those embodiments,typically the residual coke on the TLE inlet cone refractory is removedand the inlet cone refractory surface is dusted to remove residual coke.

The boron nitride paint may be applied in a single coat or in multiplecoats to achieve the desired thickness. In particular embodiment of thepresent disclosure, when applying multiple coats of the boron nitridepaint, it may be necessary to allow drying of the boron nitride paintbetween coats. The drying may be performed at ambient temperature or atan elevated temperature, depending on need. The boron nitride paint maythen be allowed to cure after completion of the application of allcoats. Curing, like drying, may be accomplished at ambient temperatureor at an elevated temperature depending on need. Typical curing timesare between 60 and 120 minutes, although more or less time may benecessary depending on such factors as the thickness of boron nitridepaint, the relative humidity, and the ambient temperature.

Following the application of the boron nitride paint, the TLE may bereassembled and placed in the discharge line of the pyrolysis furnace.It has been determined by the applicants that the boron nitride coatingoften allows for increased run times of the pyrolysis furnace betweensteam decoking. Further, it has been observed by applicants that runtimes between mechanical cleaning of the TLE can be significantlyextended as compared to run times of TLEs with uncoated tubesheets andinlet cones. While not bound by theory, applicants believe that theboron nitride coating acts to inhibit formation of catalytic coke byreducing or preventing contact between the hot product stream and themetal catalysts by coating the metal surfaces. While pyrolytic cokecontinues to form, it softer than the hard catalytic coke and may bemore easily cleaned by steam decoking. It is further believed that theboron nitride coating provides a surface with less friction than theuncoated surface, weakening coke adhesion compared to the uncoatedsurface.

While this disclosure has focused on the use of the boron nitridecoating of the TLE tubesheet and inlet cone, those of ordinary skill inthe art, with the benefit of this disclosure, will recognize that theboron nitride coating may be used with process equipment having metalsurfaces where coke forms and deposits, and where the temperature doesnot exceed 1832° F. (1000° C.) in oxidation environment and 3632° F.(2000° C.) in a vacuum or inert atmosphere. Such equipment wouldinclude, but not be limited to the pyrolysis furnace, the furnacetubing, the transfer piping from the furnace to the TLE, the pipingbetween the primary and secondary TLEs and the tubes inside the TLEs.This method could also apply to any similar process where coke is formedunder similar high temperature process conditions.

This disclosure will now be further illustrated with respect to certainspecific examples which are not intended to limit the invention, butrather to provide more specific embodiments as only a few of manypossible embodiments.

EXAMPLE 1

A previously used TLE was removed from a pyrolysis furnace process anddisassembled to allow access to the inlet tubesheet surface and TLE coneinternal components.

Coke was removed and the tubesheet and TLE inlet cone were cleaned byhydroblasting. The TLE components were allowed to dry and remaining dustwas removed.

A boron nitride paint was made by combining distilled water with a boronnitride powder with a hexagonal crystalline structure. Sufficient boronnitride powder was added to reach a concentration of 28% concentrationby weight.

A commercial spray gun was pressurized to between 20 and 40 psi usingcompressed air filtered to remove moisture and particulate. The boronnitride paint was introduced into the spray gun. A 50 micron thickcoating of boron nitride was applied to the TLE tubesheet and insidesurfaces of the TLE inlet cone. The application was made in two stageswith each stage applying a 25 micron coating. The boron nitride coatingwas allowed to dry approximately 30 to 60 minutes between coats at roomtemperature, The final boron nitride coating was cured for 60 to 120minutes at room temperature.

What is claimed is:
 1. A method of controlling coke formation inpyrolysis furnace process equipment, comprising: coating a surface ofthe process equipment with a layer of boron nitride to form a coatedsurface.
 2. The method of claim 1, wherein the boron nitride iscomprised of hexagonal crystals.
 3. The method of claim 2, wherein theboron nitride has an oxidation threshold of about 850° C. in anoxidizing atmosphere and about 2000° C. in a reducing atmosphere.
 4. Themethod of claim 1 wherein the process equipment comprises a transferline heat exchanger tube sheet.
 5. The method of claim 4 wherein theprocess equipment further comprises an inlet cone refractory of thetransfer line heat exchanger.
 6. The method of claim 5, wherein thethickness of the coated surface is between about 25 microns to about 100microns.
 7. The method of claim 6, wherein the thickness of the coatedsurface is between about 40 and about 60 microns.
 8. A method ofcontrolling coke formation in a transfer line heat exchanger,comprising: forming a boron nitride paint by combining dry boron nitridepowder with distilled water, and; applying the boron nitride paint to asurface of a transfer line heat exchanger tube sheet.
 9. The method ofclaim 8 further comprising applying the boron nitride paint to a surfaceof a transfer line heat exchange inlet refractory cone.
 10. The methodof claim 8 further comprising prior to the step of applying the boronnitride paint: roughening the surface of the transfer line heatexchanger tube sheet.
 11. The method of claim 8 further comprising priorto the step of applying the boron nitride paint: hydroblasting thesurface of the tubesheet of the transfer line heat exchanger; and dryingthe surface of the tubesheet of the transfer line heat exchanger. 12.The method of claim 8, wherein the step of applying the boron nitridepaint further comprises applying a first coat of boron nitride paint.13. The method of claim 12, wherein the step of applying the boronnitride paint further comprises applying a second coat of boron nitridepaint.
 14. The method of claim 13, wherein the step of applying theboron nitride paint further comprises, prior to the step of applying asecond coat of boron nitride paint, drying the boron nitride paint. 15.The method of claim 14, wherein the step of applying the boron nitridepaint further comprises, after the step of applying the second coat ofboron nitride paint, curing the boron nitride paint.
 16. The method ofclaim 15, wherein the boron nitride paint is cured for between 60 and120 minutes.
 17. A boron nitride paint for coating the surfaces ofpyrolysis furnace process equipment comprising: providing a dry boronnitride powder, wherein the dry boron nitride powder comprises boronnitride with a hexagonal crystalline structure; providing distilledwater; and mixing the boron nitride powder with the distilled water toform the boron nitride paint, wherein the boron nitride paint comprisesbetween 20 and 40% boron nitride by weight.
 18. The boron nitride paintof claim 17, wherein the boron nitride paint comprises between 20 and35% boron nitride by weight.